Summary

Snowflake allows a function to be invoked under a dynamically-resolved name, e.g. IDENTIFIER('my_func')(1, 2) or IDENTIFIER($fn)(). Previously the parser handled IDENTIFIER(...) itself but raised a ParseError on the trailing argument list:

ParseError: Required keyword: 'expressions' missing for <class 'sqlglot.expressions.Aliases'>

failing some of Snowflake's documented examples:

SET fn = 'add_one';
SELECT IDENTIFIER($fn)(41);
SELECT IDENTIFIER('my_func')(1, 2);
SELECT IDENTIFIER($fn)();

The aim of this PR is to add support for Snowflake's dynamic function invocation through IDENTIFIER(). DynamicIdentifier now carries an optional expressions arg holding the call arguments. The Snowflake parser consumes a trailing argument list after IDENTIFIER(...), and generation emits the call form, preserving the distinction between a name reference (IDENTIFIER('f')) and a zero-arg invocation (IDENTIFIER('f')()). Transpiling to other dialects resolves the call to a plain function call, e.g. DuckDB my_func(1, 2). Also added tests for covering the use-cases.